1. Field of the Invention
The present invention relates to a method and system for measuring and reporting time based parameters associated with heart activity. More particularly, the present invention relates to a method and system for combining information gained from an independent signal with information gained from an impedance cardiography signal for monitoring and recording signals derived from heart valve activity.
2. Discussion of the Related Art
Impedance cardiography (ICG) is a technique used to provide non-invasive monitoring and analysis of a patient's cardiac performance. ICG systems measure and report several time-based parameters related to cardiac performance, including the pre-ejection period (PEP) and the left ventricular ejection time (LVET). ICG systems produce ICG signals from monitoring movement and volume of blood as a result of the heart contracting. Exemplary ICG systems are shown and described in Ackmann et al., U.S. Pat. No. 5,178,154; and Reining, U.S. Pat. No. 5,505,209 both incorporated by reference herein in their entireties. The '154 and '209 patents disclose the use of electrode bands placed on a patient with high frequency, low magnitude electrical current applied to the electrode bands. Voltage changes across the bands are read, filtered and converted into thoracic impedance. The ICG system displays the thoracic impedance signal versus time to create a visual display or ICG waveform. The '154 patent further discloses that ICG systems can receive conventional electrocardiograph signals, signals from blood pressure monitors, signals from piezoelectric microphones attached to the chest of the patient and the like. These signals, in addition to thoracic impedance, can be stored and averaged via a memory storage device connected to the ICG system.
Phonocardiography (PCG) is a non-invasive technique used by healthcare professionals to monitor cardiac performance. PCG systems use a microphone that records sounds of heart valve activity, similar to electronic stethoscopes known in the art, in order to provide signals of acoustic events emanating from the heart. Analysis of signals recorded by PCG systems can be used to identify aortic valve opening (shown as S1 on FIG. 1) and aortic valve closing (shown as S2 on FIG. 1) of valves within a patient's heart.
Echocardiography (ECG) is another non-invasive system used to monitor heart activity. ECG uses a transducer to direct ultrasound waves into a patient's chest to produce an image of the heart muscle and heart valves. The transducer, also called a probe, is a small handheld device at the end of a flexible cable. The transducer, essentially a modified microphone, is placed against the chest and directs ultrasound waves into the chest such that some of the waves get echoed (or reflected) back to the transducer. Since different tissues and blood reflect ultrasound waves differently, these sound waves can be translated into a meaningful image of the heart that can be displayed on a monitor or recorded on paper or tape.
Still another non-invasive system used by healthcare professionals to monitor cardiac performance is a blood pressure system. A patient's blood pressure is monitored according to known techniques and converted into a blood pressure signal. The blood pressure signal is then displayed on a blood pressure waveform. Blood pressure waveforms, similar to PCG waveforms, can be used by healthcare professionals to identify heart valve closure because the dicrotic notch in blood pressure waveforms reflects closure of the aortic heart valve. Other exemplary systems using signals that have pulsatile characteristics resulting from the contraction of the heart are shown and described in Kimball et al., U.S. Pat. No. 6,763,256, herein incorporated by reference in its entirety.
The PEP is defined as the period of isovolumic ventricular contraction when the patient's heart is pumping against the closed aortic valve. In ICG systems, the PEP is measured starting with the initiation of the QRS complex (the “Q” point on FIG. 1) of the ECG signal and ending with the start of the mechanical systole as marked by the initial deflection of the systolic waveform (the “B” point on FIG. 1) of the ECG signal coincident with the opening of the aortic valve or the onset of left ventricular ejection into the aorta. The LVET begins at the end of the PEP and ends at the closure of the aortic valve (the “X” Point on FIG. 1) when ejections ends.
It is important that ICG systems provide accurate results for the PEP and the LVET because healthcare professionals utilize the results of these parameters when making decisions about patient diagnosis and care. Additionally, accurate determination of the PEP and the LVET time intervals is also required for accurate and reliable determination of subsequent and dependent parameters. For example, results from determination of the PEP and the LVET are used to calculate the systolic time ratio (STR), where STR=PEP/LVET. While many ICG systems use proprietary equations for determination of stroke volume (SV), it is commonly known that SV equations frequently incorporate LVET as an input parameter. Accordingly, accurate determination of time intervals between the PEP and the LVET is also necessary for accurate determination of SV, and subsequently for cardiac output (CO) based on SV and heart rate (HR), where CO=SV*HR.
Many ICG waveforms, particularly for healthy individuals, provide sufficient detail so that healthcare professionals can identify the location of the aortic valve opening and closing, or the LVET, with a high degree of confidence. For example, in the ICG waveform 10 depicted in FIG. 1, opening, B point, of the aortic valve and closing, X point, of the aortic valve are easily identifiable. When comparing the ICG waveform 10 with the phonocardiograph (PCG) waveform 12 (both shown in FIG. 1), marking of the B point in the ICG waveform 10 is confirmed by the time-associated presence of the S1 component in the PCG waveform 12. Similarly, marking of the X point in the ICG waveform 10 is confirmed by the time associated presence of the S2 component in the PCG waveform 12.
Traditionally, ICG systems only analyze attributes of the impedance signal when determining the location of heart valve activity. Some ICG systems may record and display PCG signals, blood pressure signals, and/or other signals having pulsatile characteristics resulting from contraction of the heart, but these ICG systems do not integrate the signals into an automatic location of heart valve activity. ICG systems alone often lack sufficient information for healthcare professionals to accurately and reliably determine the PEP and the LVET because of confounding information related to opening and closing of the patient's aortic valve. For example, in the ICG waveform 10 depicted in FIG. 2, closure, X point, of the aortic valve could be any of several depressions following the peak blood flow, C. The known algorithm selected the deepest depression in the ICG waveform 10 because the aortic valve closure is often thought to produce the strongest negative signal. However, when the ICG waveform 10 depicted in FIG. 2 is compared with the PCG waveform 12 depicted in FIG. 2, the aortic valve closure, X point, should have been one of the later depressions in the ICG waveform 10 in order to correlate with the time associated presence of the S2 component in the PCG waveform 12. Accordingly, there is a need for a method and system for measuring and reporting time based parameters associated with heart activity that correlates impedance signals from ICG systems with independent signals derived from heart valve activity in order to provide more accurate identification of heart valve activity.
It is known that experienced healthcare professionals can recognize, or diagnose, certain disease states by analyzing hemodynamic parameters and ICG waveforms provided by some ICG systems. Experienced healthcare professionals can easily recognize the systolic and diastolic segments of ICG waveforms in addition to other attributes of the waveform, such as amplitude, shape, tone, slope and timing, in combination with hemodynamic parameters. Analysis of these attributes allows experienced healthcare professionals to ascertain an underlying disease state.
It is also known that some ICG systems provide minimal ICG waveform information. When using these types of systems, healthcare professionals must rely largely on numeric parameters to make a diagnosis because these systems do not provide other information. With ICG systems that display waveforms but provide them individually, experienced healthcare professionals may still be unable to analyze all waveform attributes and relationships to make a diagnosis. Accordingly, there exists a need for an improved method for displaying ICG waveform information in combination with information obtained from independent signals.
Based on the foregoing, there exists a need for a method and system that provides better identification of heart valve activity when measuring cardiac function within an ICG system. There also exists a need for method and system for measuring and reporting time based parameters associated with heart activity that correlates impedance signals from ICG systems with independent signals derived from heart valve activity in order to provide more accurate identification of heart valve activity. There exists yet another need for an improved method for displaying waveform information in combination with information obtained from independent signals.